Data Sheet
January 2000
L7556, L7557 Low-Power SLICs
with Battery Switch
Features
s
Description
These electronic subscriber loop interface circuits
(SLICs) are optimized for low power consumption
while providing an extensive set of features.
The SLICs include an auxiliary battery input and a
built-in switch. In short-loop applications, they can be
used in high battery to present a high on-hook volt-
age, and then switched to low battery to reduce off-
hook power.
The SLICs also include a summing node for meter
pulse injection to 2.2 Vrms. A spare, uncommitted op
amp is included for meter pulse filtering.
The switched battery is applied to the power amplifi-
ers of the device. There are two versions. The L7556
has the battery switch completely under processor
control. The L7557 can automatically switch to lower
battery when appropriate and includes hysteresis to
avoid frequent switching. To make the switch silent,
an external capacitor can be added to slow the tran-
sition.
The L7556 is suited for applications serving only
short loops, where a high on-hook voltage is required
for compatibility with preexisting standards.
The L7557 is suited for applications where a full loop
range is needed, but low short-loop power is desired.
It is a much lower-cost solution than a switching reg-
ulator, and also occupies much less PCB area, need-
ing only a battery filter capacitor and a diode for
implementation.
The device is available in a 32-pin PLCC package. It
is built by using a 90 V complementary bipolar inte-
grated circuit (CBIC) process.
Auxiliary input for second battery, and internal
switch to enable its use to save power
Low active power (typical 125 mW during on-hook
transmission)
Supports meter pulse injection
Spare op amp for meter pulse filtering
–16 V to –60 V power supply operation
Distortion-free on-hook transmission
Convenient operating states:
— Forward powerup
— Disconnect (high impedance)
— 2-wire wink (zero loop voltage)
Adjustable supervision functions:
— Off-hook detector with longitudinal rejection
— Ground key detector
— Ring trip detector
Independent, adjustable, dc and ac parameters:
— dc feed resistance
— Loop current limit
— Termination impedance
Thermal protection
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L7556, L7557 Low-Power SLICs
with Battery Switch
Data Sheet
January 2000
Table of Contents
Contents
Page
Figures
Page
Features ..................................................................... 1
Description .................................................................. 1
Pin Information ............................................................ 4
Functional Description ................................................. 6
Absolute Maximum Ratings ........................................ 6
Recommended Operating Conditions ......................... 7
Electrical Characteristics ............................................. 7
Ring Trip Requirements ......................................... 11
Test Configurations .................................................. 12
Applications .............................................................. 14
Design Considerations ........................................... 16
Characteristic Curves............................................. 17
dc Applications ....................................................... 20
Battery Feed......................................................... 20
Switching the Battery............................................ 20
Overhead Voltage ............................................... 21
Adjusting Overhead Voltage ................................ 21
Adjusting dc Feed Resistance.............................. 22
Loop Range.......................................................... 22
Off-Hook Detection .............................................. 22
Ring Trip Detection.............................................. 23
Ring Ground Detection........................................ 23
ac Design ............................................................... 24
First-Generation Codecs ..................................... 24
Second-Generation Codecs ................................ 24
Third-Generation Codecs .................................... 24
Selection Criteria ................................................. 24
PCB Layout Information ............................................ 26
Outline Diagram......................................................... 27
32-Pin PLCC ........................................................... 27
Ordering Information.................................................. 28
Tables
Page
Table 1. Pin Descriptions ............................................ 4
Table 2. Input State Coding ........................................ 6
Table 3. Supervision Coding ....................................... 6
Table 4. Power Supply ................................................ 7
Table 5. 2-Wire Port .................................................... 8
Table 6. Analog Pin Characteristics ............................ 9
Table 7. Uncommitted Op Amp Characteristics .......... 9
Table 8. ac Feed Characteristics .............................. 10
Table 9. Logic Inputs and Outputs ............................ 11
Table 10. Parts List for Loop Start and Ground
Start Applications ...................................... 15
Table 11. 600
Ω
Design Parameters ......................... 16
Figure 1. Functional Diagram ..................................... 3
Figure 2. Pin Diagram (PLCC Chip) ........................... 4
Figure 3. Ring Trip Circuits ....................................... 11
Figure 4. Basic Test Circuit .......................................12
Figure 5. Longitudinal Balance .................................12
Figure 6. Longitudinal PSRR ....................................13
Figure 7. RFI Rejection .............................................13
Figure 8. Longitudinal Impedance ............................13
Figure 9. Metallic PSRR ...........................................13
Figure 10. ac Gains ..................................................13
Figure 11. Basic Loop Start Application Circuit
Using T7504 Type Codec ........................14
Figure 12. Ring Ground Detection Circuit .................14
Figure 13. Receive Gain and Hybrid Balance vs.
Frequency ...............................................17
Figure 14. Transmit Gain and Return Loss vs.
Frequency ...............................................17
Figure 15. Typical V
CC
Power Supply Rejection .......17
Figure 16. Typical V
BAT
Power Supply
Rejection .................................................17
Figure 17. Loop Closure Program Resistor
Selection ..................................................18
Figure 18. Ring Ground Detection Programming .....18
Figure 19. Loop Current vs. Loop Voltage ................18
Figure 20. Loop Current vs. Loop Resistance ..........18
Figure 21. Typical SLIC Power Dissipation vs.
Loop Resistance ......................................19
Figure 22. Power Derating ........................................19
Figure 23. Longitudinal Balance Resistor Mismatch
Requirements ..........................................19
Figure 24. Longitudinal Balance vs. Protection
Resistor Mismatch ...................................19
Figure 25. Loop Current vs. Loop Voltage ................20
Figure 26. SLIC 2-Wire Output Stage .......................21
Figure 27. Equivalent Circuit for Adjusting the Over-
head Voltage ...........................................21
Figure 28. Equivalent Circuit for Adjusting the dc
Feed Resistance ......................................22
Figure 29. Adjusting Both Overhead Voltage and dc
Feed Resistance .....................................22
Figure 30. Off-Hook Detection Circuit
Applications .............................................22
Figure 31. Ring Trip Equivalent Circuit and
Equivalent Application .............................23
Figure 32. ac Equivalent Circuit Not Including Spare
Op Amp ...................................................25
Figure 33. ac Equivalent Circuit Including Spare
Op Amp ...................................................25
2
Lucent Technologies Inc.
Data Sheet
January 2000
L7556, L7557 Low-Power SLICs
with Battery Switch
Description
(continued)
BGND
AGND
V
BAT2
V
BAT1
I
PROG
L
BAT
BS1
BS2
V
CC
BS
BATTERY
SWITCH
POWER CONDITIONING
& REFERENCE
V
REG
CF1
CF2
DCOUT
+
1 V/8 mA
VITR
–
SN
PT
A=4
SPARE
OP AMP
–
XMT
+
–
PR
A = –4
RCVN
RCVP
+
B0
DCR
dc RESISTANCE
ADJUST
BATTERY FEED
STATE CONTROL
B1
LCTH
LOOP CLOSURE DETECTOR
+
NLC
–
RTSP
RTSN
RING TRIP DETECTOR
+
NRDET
–
RGDET
ICM
RING GROUND
DETECTOR
12-2551.a (F)
Figure 1. Functional Diagram
Lucent Technologies Inc.
3
L7556, L7557 Low-Power SLICs
with Battery Switch
Data Sheet
January 2000
Pin Information
V
BAT2
I
PROG
NC
L
BAT
BS1
BS2
31
4
V
CC
RCVP
RCVN
LCTH
DCOUT
V
BAT1
PR
CF2
CF1
5
6
7
8
9
10
11
12
13 14
VITR
3
2
1
32
30
29
28
27
26
SN
XMT
B1
NLC
NRDET
RTSP
RTSN
PT
BGND
32-PIN PLCC
NC
25
24
23
22
20
DCR
21
15
ICM
BS
16
RGDET
17
B0
18
AGND
19
AGND
12-2548.q (F)
Figure 2. Pin Diagram (PLCC Chip)
Table 1. Pin Descriptions
Pin
1
2
3
4
4
5
6
7
8
9
10
Symbol Type
Description
V
BAT2
—
Auxiliary Battery Supply.
Negative high-voltage battery, lower in magnitude than
V
BAT1
, used to reduce power dissipation on short loops.
I
PROG
I
Current-Limit Program Input.
A resistor to DCOUT sets the dc current limit of the
device.
BS
I
Battery Switch.
See Table 2 for description.
NC
—
No Connection (L7556 Only).
Do not use as a tie point.
L
BAT
O
Lower Battery in Use (L7557 Only).
When high, this open-collector output indicates
the device has switched to V
BAT2.
To use, connect a 100 kΩ resistor to V
CC
.
V
CC
—
+5 V Power Supply.
RCVP
I
Receive ac Signal Input (Noninverting).
This high-impedance input controls the ac
differential voltage on tip and ring.
RCVN
I
Receive ac Signal Input (Inverting).
This high-impedance input controls the ac differ-
ential voltage on tip and ring.
LCTH
I
Loop Closure Threshold Input.
Connect a resistor to DCOUT to set off-hook thresh-
old.
DCOUT
O
dc Output Voltage.
This output is a voltage that is directly proportional to the absolute
value of the differential tip/ring current.
V
BAT1
—
Battery Supply.
Negative high-voltage power supply, higher in magnitude than V
BAT2
.
4
Lucent Technologies Inc.
Data Sheet
January 2000
L7556, L7557 Low-Power SLICs
with Battery Switch
Pin Information
(continued)
Table 1. Pin Descriptions
(continued)
Pin
11
12
13
14
15
Symbol Type
Description
PR
I/O
Protected Ring.
The output of the ring driver amplifier and input to loop sensing cir-
cuitry. Connect to loop through overvoltage protection.
CF2
—
Filter Capacitor 2.
Connect a 0.1 µF capacitor from this pin to AGND.
CF1
—
Filter Capacitor 1.
Connect a 0.47 µF capacitor from this pin to pin CF2.
VITR
O
Transmit ac Output Voltage.
This output is a voltage that is directly proportional to the
differential tip/ring current.
ICM
I
Common-Mode Current Sense.
To program ring ground sense threshold, connect a
resistor to V
CC
and connect a capacitor to AGND to filter 50/60 Hz. If unused, the pin
can be left unconnected.
RGDET
O
Ring Ground Detect.
When high, this open-collector output indicates the presence of
a ring ground. To use, connect a 100 kΩ resistor to V
CC
.
B0
I
State Control Input.
B0 and B1 determine the state of the SLIC. See Table 2.
AGND
—
Analog Signal Ground.
AGND
—
Analog Signal Ground.
DCR
I
dc Resistance for Low Loop Currents.
Leave open for dc feed resistance of 115
Ω,
or short to DCOUT for 615
Ω.
Intermediate values can be set by a simple resistor
divider from DCOUT to ground with the tap at DCR.
BGND
—
Battery Ground.
Ground return for the battery supply.
PT
I/O
Protected Tip.
The output of the tip driver amplifier and input to loop sensing circuitry.
Connect to loop through overvoltage protection.
RTSN
I
Ring Trip Sense Negative.
Connect this pin to the ringing generator signal through a
high-value resistor.
RTSP
I
Ring Trip Sense Positive.
Connect this pin to the ring relay and the ringer series resis-
tor through a high-value resistor.
NRDET
O
Ring Trip Detector Output.
When low, this logic output indicates that ringing is tripped.
NLC
O
Loop Detector Output.
When low, this logic output indicates an off-hook condition.
B1
I/O
State Control Input.
B0 and B1 determine the state of the SLIC. See Table 2. Pin B1
has a 40 kΩ pull-up. It goes low in the event of thermal shutdown.
XMT
O
Transmit ac Output Voltage.
The output of the uncommitted operational amplifier.
SN
I
Summing Node.
The inverting input of the uncommitted operational amplifier. A resis-
tor or network to XMT sets the gain.
NC
—
No Connection.
Do not use as a tie point.
BS2
—
Battery Switch Slowdown.
A 0.1 µF capacitor from BS1 to BS2 will ramp the battery
switch transition for applications requiring quiet transition. If not needed, the pin can be
left open.
BS1
—
Battery Switch Slowdown.
A 0.1 µF capacitor from BS1 to BS2 will ramp the battery
switch transition for applications requiring quiet transition. If not needed, the pin can be
left open.
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
Lucent Technologies Inc.
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